Typical redundancy repair uses metal fuses that require extra die space for the programming pads and cannot be programmed after the devices have been packaged. Other redundancy schemes run into difficulties when the programming voltages approach the breakdown limits of the source/drain junctions or gate oxides of the bulk devices.
A significant percentage of the cost of commodity memories is that of packaging and burn-in testing (after packaging). During burn-in, parts that were good after packaging fail for many reasons. Many of the parts fail due to the failure of individual memory bits that could be repaired if caught at wafer probe. The ability to repair failures at burn-in (or at the end-user's facility) would significantly reduce the loss at burn-in, and reduce the complexity of testing required at wafer probe (testing each die at the wafer level).